Some of the casting defects are described below:
1. Surface Roughness:
Too coarse a moulding sand or when pouring temperature is high leads to rough or pebbly surfaces on castings. In steel castings, roughness is produced due to occurrence of surface reaction at mould-metal interface in which iron is oxidised and iron oxide reacts with silica to forms rough compound. Surface reactions sometimes also cause sub-surface porosity or pin holes.
2. Scabs or Buckles:
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These defects occur due to some sand shearing from the cope surface and as a result there being a layer of metal separated from the casting proper by a layer of sand. Scabs are relatively small particles and buckles are big defects.
These occur due to use of too fine a sand, low permeability, high moisture, uneven ramming of mould, low or intermittent running of molten metal over the sand surface. These can be avoided by using sand with high hot plasticity or low expansion characteristics, using expansion buffer in sand, and rapidly filling the mould.
3. Blow-Holes:
They take the form of internal voids (smooth, round or oval holes with a shiny surface), dispersed internal porosity or surface depression as a result of excessive gaseous materials that cannot escape.
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They are caused by hard ramming, excessive moisture, low permeability, excessive fine grains, incomplete or improper venting, low temperature of mould and excessive carbonaceous or other organic materials (gas producing ingredients). It is sometimes caused by bad patterns and core-box arrangements, which lead to trapping of gases in blind places in the mould. They can be controlled by taking care of the above points.
4. Pinholes:
Surface reactions sometimes cause subsurface porosity or pinholes. In aluminium alloys containing more than about 1% magnesium-magnesium react with water vapour of the mould to form H2 gas resulting in hydrogen filled pinholes at the surface. In steel castings, subsurface pinholes may result from incomplete de-oxidation of the molten metal.
5. Sand Spots:
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These appear as irregularly shaped depressions spaced randomly or clustered on casting and are due to impurities collected at one or more vertices developed by the metal. Sand spots are caused by the metal washing particles from the runner system or mould walls, by excess turbulence in gating system, and by spurting of metal into the mould. These can be controlled by adopting proper moulding, gating and melting techniques.
6. Swell:
It refers to the condition of enlargement of mould cavity when the molten metal is poured into the mould. It is caused, either by insufficient ramming or by pouring the metal too rapidly.
7. Shrinkage:
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It refers to the condition of voids in the casting resulting from concentrated contraction of the metal during solidification. It may be caused by improper location of gates and runners, poor design and inadequate filleting of corners.
8. Hot-Tears:
There are the cracks having ragged edges due to tensile stresses during solidification. It is due to the discontinuity in the metal casting resulting from hindered contraction, occurring just after the metal has solidified.
It is caused by excessive mould hardness of ramming, high dry and hot strength, improper metallurgical and pouring temperature controls, and provision of insufficient fillets or brackets at the junctions of sections.
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9. Cold-Cracks:
These are similar to hot tears except that discontinuity is less and defect occurs below 270°C.
10. Cold-Shots or Surface Laps:
These are external defects caused by two streams of metals that are too cold to fuse properly; these can occur due to slow pouring, poor design and small gate; and can be controlled by the use of hotter metal using streamlined spines to give smoother flow. In this defect, small shot-like spheres of metal are almost distinct from casting.
11. Lifts and Shifts:
They are external defects in castings caused due to misalignment of pattern parts, flask equipment, poor fitting of mould jackets and improper handling of moulds.
12. Sponginess or Honey-Combing:
It is also an external defect, consisting of a number of cavities in close proximity. It is caused by dirt or swarm held in the molten metal, imperfect skimming and poor quality of molten metal.
13. Displaced Cores:
These occur due to the buoyancy of cores in molten metal. Cores should be firmly anchored. In long cores, bending can be taken care of by using stiff core irons, and chaplets placed correctly.
14. Misplaced Cores:
These result in unequal thickness of casting and occur due to moulder not checking up the various thicknesses, when finally assembling the mould and cores.
15. Pour-Short:
It refers to the condition of incomplete filling of mould due to insufficient metal in the ladle and interruptions during pouring operation.
16. Gas Porosity:
These are the rounded voids with smooth walls and occur due to gases dissolved in metal during melting and pouring. Imperfect feeding causes angular voids with dendrite arms protruding into the voids. Fine micro-porosity is observed in non-ferrous metals and occurs due to gas content and metal shrinkage.
17. Run-Outs:
Drainage of metal from the cavity is called run-out. It gives incomplete casting and is caused by too large pattern, uneven match plate surfaces, inadequate mould weights and clamps, and excessive pouring pressure.
18. Metal Penetration:
It refers to the condition of penetration of metal in the interstices of the sand grains. It causes a fused aggregate of metal and sand on the surface of casting which results in rough surface finish. It is caused by soft ramming, too coarse mould and core sand, and excessive metal temperature.
19. Fins:
A thin projection of metal not intended as a part of casting is called fin. Fins usually occur at the parting of mould and core section. These are caused by run out of metal, poor fittings of moulds and cores, high metal pressure, and insufficient weights and clamps.
20. Internal Air Pockets:
These are caused by pouring boiling metal or rapid pouring of molten metal in the mould.
21. Dross or Sand Inclusion:
These are oxides of other reaction products of metal being cast and these should be removed from the ladle before pouring metal. These defects are caused by improper control of melting and pouring, gating design and moulding sand practice.
Slag or dross inclusions can be prevented from entering from the ladle, by skimming before pouring or using bottom-pouring ladles, employing pouring basins so that any slag or dross entering from the ladle will rise and not pass into the runner system, designing the runner system to exert additional skimming action on the flowing metal, streamlining the runner system will minimise any tendency to entrap air or form dross or slag inclusions during pouring.
Careful control of mould permeability and gas content reduce danger of entrapping air or mould gases and minimize any tendency to form dross from mould-metal reactions.
22. Misruns:
Misruns may be present in the form of improperly filled corners and mould cavities. These occur because of low pouring temperature, lack of fluidity of the metal, too small gates, too many restrictions in gating system etc. Another defect called the cold shot occurs when two cold streams of molten metal meet at the junction of a mould cavity and do not fuse together and thus the mould is not properly filled with metal.
23. Pinholes and Gas Holes:
Pin holes are numerous, very small holes visible on the surface of casting after it has been cleaned by shot-blasting. Pinholes are caused by high- moisture and gas-producing material in sand due to faulty metal. Gas holes appear when the metal is machined or cut into sections. These also occur due to moisture and faulty metal.
24. Seams:
These refer to defects at the junctions of two streams of metal.
25. Distortion:
It occurs due to contraction stresses.
26. Drawing:
It results in minute or fairly large holes with a black surface. It is associated with the contraction of metal in the mould. It is common with thick bosses which remain liquid after the surrounding metal has solidified. Such portions should be surrounded by chills to promote rapid cooling.
27. Some other minor casting defects are drops, crushes, cuts and washes. Drop occurs when the upper surface of mould cracks and pieces of sand fall into the molten metal. Drop may occur due to low green strength, low mould hardness, using hot sand, insufficient reinforcement.
Defects Resulting from Incomplete Feeding:
Solidification shrinkage is the biggest cause for many of casting defects.
External Imperfections:
These appear in the form of localized cavities at unfed hot spots in the casting (depressed regions on cope and upper surfaces).
Whenever feeding is grossly inadequate, internal unsoundness is usually indicated by some external imperfection, wall punctures, deformation by dishing at the weakest point, elongated wormholes appearing at the riser on cope surfaces, defects resembling collections of dross where cope surfaces are wrinkled and drawn inward, and small voids in the form of pin holes.
Internal Imperfections:
Like gross shrinkage, centre-line shrinkage, micro-porosity results from solidification shrinkage:
a. Centre-Line Shrinkage:
Centre-line shrinkage is a narrow, more or less continuous void sometimes found along the centre line of castings with extensive plate like sections.
This defect is found only in alloys like steel which freeze over a relatively narrow temperature range.
b. Interdendritic Shrinkage (Micro Porosity):
Alloys that freeze over a wider temperature interval tend to exhibit this defect when improperly fed and also due to dissolved gases.
c. Internal Hot Tearing:
Internal hot tearing occurs due to improper feeding. ‘Internal hot tears’ are radially disposed discontinuities inside castings, emanating from low density area. These are disclosed by radiography. The discontinuities resemble external hot tears, except that they are radial rather than roughly parallel.